1. Field of the Invention
This invention relates to the cleaning of fluids, such as for the removal of oil and other contaminants from produced water that results from a hydrocarbon production operation.
2. Description of Related Art
With the production of crude oil and other hydrocarbons, there is normally an associated aqueous stream of produced water that results, the produced water typically including a variety of contaminants, e.g., dissolved and/or suspended materials such as one or more of the following: oil or other hydrocarbons; minerals, such as calcium, boron, carbonates, chlorides, hydroxides, sulfates, iron, magnesium, sodium, silicates, and nitrates; organic materials, such as formate, acetate, propionate, butyrate, and valerate; and the like. The contaminants can include solid, liquid, or gaseous phases, typically of material that is immiscible in the fluid. There is an increasing interest in facilitating the removal of contaminants from the produced water and reducing the quantities of contaminants that are released to the environment by or during the disposal of this stream. In some cases, there is an increased interest in removing both the suspended solids and dissolved hydrocarbon (or other materials that may feed organisms in the water) so that this water can be reinjected to enhance oil recovery with less damage to the reservoir than untreated water.
Conventional methods for treating produced water and other contaminated fluids to remove contaminants therefrom include the use of settling and filtering operations and chemical additives (e.g., ion exchange resin beds, scavengers, adsorption processes). For example, in one conventional method, a stream of produced water is passed through a separation vessel, where hydrocarbon particles in the water can rise to the surface. The time required for separation in a separation vessel depends on such factors as the size of the hydrocarbon particles, the temperature of the fluid, the character of the flow, and the like. An effective separation operation can require a large vessel and, when reasonable separation times are used, typically does not result in separation of small hydrocarbon particles, such as those having a diameter less than about 100 micron, which tend to rise or settle slowly.
Another conventional method for removing small oil particles from an aqueous stream includes the use of chemicals and gas flotation aids, such as bubbles, as described in U.S. Pat. No. 5,543,043. Chemical polymer can also be added to increase the efficiency of clarification. Polymers that may be used are typically of high molecular weight, long branched chain with many charged side branches. These charged side branches attract charged particles such as oil and suspended solids. Assisted by gas flotation, the polymer and entrained particles migrate to the surface of the water, thereby forming a foaming floc that can be removed by skimming the surface. The oily floc is normally not economical to recover and must be disposed of as a waste. For a dispersed gas flotation unit, the mean generated gas bubble size can typically range from about 80 to 100 micrometers. Even if the bubbles are optimally sized, the bubbles tend to rise, collide, and combine to form larger bubbles, which are less effective in removing the contaminants.
While conventional methods have proven successful for removing at least some contaminants, a continued need exists for improved methods and systems for removing contaminants from produced water and other contaminated fluids.